A mobile terminal such as a smart phone has a vibration function (or, a haptic function) for not only interfacing the receipt of a call but also interfacing for feeding back a key input, an occurrence of an event, execution of an application or the like to a user.
A vibration motor (or, a vibration generating apparatus) implementing such a vibration function converts energy obtained by an electromagnetic action into a vibration energy and is generally classified into a linear-type vibration motor and a flat/coin-type vibration motor depending on an operating way or pattern.
The flat/coin-type vibration motor has a behavior characteristic in which an inertia remains due to rotation, and thus a linear vibration generating apparatus having no rotation inertia is generally used when a rapid response speed is demanded.
An existing linear vibration generating apparatus (or, a linear-vibrating motor) 500 as described above includes a vibrating body 520 having a magnet 521 and a weight 523 surrounding an outer portion of the magnet 521, and an elastic body (e.g., a spring) 540 for physically supporting the vibrating body 520 in a vertical direction, as shown in FIG. 1.
In addition, as shown in FIG. 1, a fixed body 530 is located at a central axis provided to a middle portion of the linear vibration generating apparatus 500, and the fixed body 530 includes a yoke 533 and a coil 531 provided at the yoke 533 to generate an electromagnetic force to the magnet 521.
The linear vibration generating apparatus 500 is designed so that an electromagnetic force generated at the coil 531 and a physical elastic force provided by the elastic body 540 have resonance to each other. If a power of a specific frequency with a time-variant characteristic is applied to the coil 533 to generate an electromagnetic force, the generated electromagnetic force interacts with the elastic force of the elastic body 540 so that the vibrating body 520 makes a linear movement in a vertical direction. As shown in FIG. 1, the linear vibration generating apparatus 500 may further include a connection unit 570 and a circuit board 560 for interfacing with an external power or a control signal.
The existing linear vibration generating apparatus 500 includes a damper 550 for relieving a physical impact applied to a case or housing 510 which forms an inner space, and the damper 550 is provided at a location opposite to the magnet 521 to relieve or absorb an impact generated when the vibrating body 520 moves upwards, thereby minimizing the occurrence of unnecessary noise.
In this regard, a fluid-type damper having viscosity may also be used at the upper surface of the magnet 521 of the existing linear vibration generating apparatus 500, and in this case, a magnetic fluid (MF) 70 is frequently used as the fluid-type damper with viscosity in order to enhance a binding force. The magnetic fluid is formed by dispersing metal powder such as oxidized steel magnetized to a liquid base oil into a collide form, and a surfactant may also be added thereto depending on situations.
The magnetic fluid 70 has physical characteristics of fluid and thus more smoothly relieves a physical impact between components, and also the magnetic fluid 70 has magnetism to keep a fixed location without deviating from the location due to the magnetic force with the magnet 521.
The damper 550 is made of material such as rubber, silicon, foam rubber, Poron, foaming resin or the like in order to absorb an impact, and as shown in FIG. 2, the damper generally has a disk shape to include a shape corresponding to an outer circumferential portion of the magnet 521.
In order to absorb an impact and minimize the occurrence of unnecessary noise, the existing linear vibration generating apparatus 500 is used together with the damper 550 and the (magnetic fluid) viscous fluid 70, and it has been found as a result of experiments and observations that unexpected noise is generated from the linear vibration generating apparatus 500 used together with the damper 550 and the viscous fluid (magnetic fluid) 70.
The vibrating body 520 makes a linear movement in a vertical direction at a high speed due to the electromagnetic force generated at the coil 531 and the elastic force of the elastic body 540, and when the vibrating body 520 moves to a maximum upward displacement, the viscous fluid 70 provided at (or, applied to) the upper surface of the magnet 521 makes a surface contact with the damper 550. At this time, the viscous fluid 70 is entirely closely adhered to the lower surface of the damper 550 simultaneously as shown in a right part of FIG. 2. In the right part of FIG. 2, the portion A is a region where the viscous fluid 70 makes a surface contact with the damper 550.
The housing (case) may be made of metal material to shield an external magnetic force or the like, and thus the magnetic fluid 70 having magnetism due to the viscous fluid may form a magnetic force with respect to the housing. In addition, since the magnetic fluid 70 has physical characteristics of fluid (viscosity or the like), if the vibrating body 520 moves downwards, the magnetic fluid 70 closely adhered to the damper 550 and facing the damper 550 does not instantly move downwards together with the vibrating body 520 (specifically, the magnet) but keeps a state of being adhered to the damper 550, during an extremely short time or more.
In addition, if an existing damper is used, a space or gap is not present between the magnetic fluid 70 and the damper 550, and thus the magnetic fluid 70 makes a surface contact with the damper 550 as a whole so that an interface of the magnetic fluid 70 and the damper 550 is perfectly sealed.
Due to this phenomenon, while the vibrating body 520 moves downwards, the magnetic fluid 70 may form a pillar-like shape with which a center portion of the magnetic fluid 70 gradually narrows as shown in FIG. 3, or while the vibrating body 520 moves to a maximum downward displacement, the center portion of this fluid pillar may be fractured. During this process, unnecessary noise is generated.
In addition, the magnetic fluid 70 facing the damper 550 is adhered to the damper 550 for a predetermined time, and while the vibrating body 520 moves down to a maximum displacement, a downward pulling force gradually increases, and the sealed interface of the magnetic fluid 70 is separated, thereby causing unnecessary noise.